The use of compounds comprising bicyclo-orthoester groups in coating compositions is known from U.S. patent publication No. U.S. Pat. No. 4,338,240. In this patent publication the use and the preparation of bicyclo-orthoester-functional compounds (hereinafter bicyclo-orthoester will be abbreviated to BOE) is described. Described are, e.g., BOE-functional compounds, which are the adduct of two compounds comprising one hydroxyl group and one BOE group and one compound comprising two isocyanate groups. The compounds are cross-linked by means of cationic ring opening homopolymerisation of the BOE groups. In that case, however, the presence of moisture has to be excluded. Furthermore, energy in the form of ultraviolet, infrared or microwave irradiation or heat has to be supplied during the polymerisation process.
In an earlier patent application WO 97/31073, a coating composition is described comprising a first compound comprising at least one BOE group and a second compound comprising at least two hydroxyl-reactive groups.
It has now been found that a coating composition comprising a compound comprising at least one BOE group and at least one other functional group has similar properties as the coating compositions covered by the above-mentioned earlier patent application WO 97/31073.
A coating composition comprising a compound comprising at least one BOE group is a composition having latent hydroxyl groups. In the presence of water or moisture from the air the BOE groups will be hydrolysed, forming hydroxyl groups. This reaction is also known as deblocking. During deblocking few if any volatile components are released. When the BOE group is deblocked in this manner, it is not possible to obtain a homopolymer of BOE groups by cationic polymerization. However, when hydroxyl-reactive groups are present in the coating composition, the deblocked hydroxyl groups can react with the hydroxyl-reactive groups to give a cross-linked polymer.
The use of compounds comprising BOE groups in coating compositions has several advantages over the use of compounds having free hydroxyl groups, such as hydroxyl-functional reactive diluents, hydroxyl-functional main binders, e.g. polyester polyols and acrylate polyols, and even compounds where the BOE groups have already been hydrolysed.
Firstly, the viscosity of compounds comprising BOE groups is lower than that of the corresponding hydrolysed compounds. In consequence, less viscosity-reducing solvent which evaporates in air is needed in the coating composition.
Secondly, because of the stability of the BOE-functional compounds the pot life: drying time ratio of compositions according to the invention is particularly favourable, for hydrolysis only takes place in the presence of water or moisture.
Thirdly, in coating compositions of the present invention BOE-functional compounds have the advantage that hydrolysis of the BOE group produces a substantial increase in the composition's viscosity. A high viscosity will give reduced sagging of the coating composition on the substrate.
Finally, it has been found that the coating compositions of the present invention provide a high build behavior.